1. Field of the Invention
The present invention is directed generally to acoustic imaging and more particularly the direct imaging of an object utilizing the method of acoustical holography whereby the object being imaged can be in close proximity to the hologram surface.
2. Description of the Related Art
Holography involves combining or interfering an object wave or energy with a reference wave or energy to form an interference pattern referred to as the hologram. A fundamental requirement for the forming of the hologram and the practice of holography is that the initial sources of the object wave and reference wave or energy are coherent with respect to the other wave. That is to say that all parts of both the object wave and the reference wave are of the same frequency and of a defined orientation, namely, a fixed spatial position and angle between the direction of propagation of the two sources. When performing holography the object wave is modified by interference with the structure within the object of interest. As this object wave interacts with all points of the object in the path of the wave, the three-dimensional features of the object impart identifying phase and amplitude changes on the object wave. Since the reference wave is an unperturbed (pure) coherent wave, its interference with the object wave results in an interference pattern that identifies the 3-D positioning and characteristics (ultrasonic absorption, diffraction, reflection, and refraction) of the scattering points of the object.
A second process, (the reconstruction of the hologram) is then performed when a coherent viewing source (usually light from a laser) is transmitted through or reflected from the hologram. The hologram pattern diffracts light from this coherent viewing or reconstruction source in a manner to faithfully represent the 3-D nature of the object, as seen by the ultrasonic object wave.
Thus, traditionally, to perform holography, coherent wave sources are required. This requirement currently limits practical applications of the practice of holography to the light domain (e.g., a laser light) or the domain of acoustics (sometimes referred to as ultrasound due to the practical application at ultrasonic frequencies) as these two sources are currently the only available coherent energy sources. Thus, further references to holography or imaging system will refer to the through-transmission holographic imaging process that uses acoustical energies usually in the ultrasonic frequency range and more specifically from 1 to 10 MHz. Alternatively, higher or lower frequencies would also apply.
In the practice of ultrasound holography, one key process is the generation of the ultrasound, such as by a large area coherent ultrasound transducer. A second key process is the projection of the object wave information from a specific volume within the object into the hologram detection plane by means of the ultrasonic lens projection system. A third key process is the detection and reconstruction of the ultrasonic hologram into visual or useful format.
Although other configurations can be utilized, a common requirement of the source transducers for both the object and reference waves is to produce a large area plane wave having constant amplitude across the wave front and having a constant frequency for a sufficient number of cycles to establish coherence. Such transducers will produce this desired wave if the amplitude of the ultrasound output decreases in a Gaussian distribution profile as the edge of the large area transducer is approached. This decreasing of amplitude as the edge is approached, reduces or eliminates the “edge effect” from the transducer edge, which would otherwise cause varying amplitude across the wave front as a function distance from the transducer.
In the process of through-transmission ultrasonic holographic imaging, the pulse from the object transducer progresses through the object, then through a focusing lens system and at the appropriate time, the pulse of ultrasound is generated from the reference transducer such that the object wave and reference wave arrive at the detector at the same time to create an interference pattern (i.e., the hologram). For broad applications, the transducers need to be able to operate at a spectrum or bandwidth of discrete frequencies. Multiple frequencies allow comparisons and integration of holograms taken at selected frequencies to provide an improved image of the subtle changes within the object.
A hologram can also be formed by directing the object wave through the object at different angles to the central axis of the lens system. This is provided by either positioning or rotating the object transducer around the central axis of the lens system by using multiple transducers positioned such that the path of transmission of the sound is at an angle with respect to the central axis of the lens system.
With a through-transmission imaging system, it is important to determine the amount of resolution in the “z” dimension that is desirable and achievable. Since the holographic process operates without limits of mechanical or electronic devices to detect and form the image, but rather reconstructs images from wave interactions, the resolution achievable can approach the theoretical limit of ½ the wavelength of the ultrasound used. However, the amount of information displayed for the user in this situation may be too great. It may be desirable to limit the “z” direction image volume so that on can “focus” in on one thin volume slice and thereby reduce the amount of data. Thus, it is of value to develop a means for projecting a planar slice within a volume into the detector plane. One such means is a large aperture ultrasonic lens system that will allow the imaging system to “focus” on a plane within the object. Additionally, this lens system and the corresponding motorized computer controlled lens drive will allow one to adjust the focal plane and at any given focal plane to be able to magnify or demagnify at a selected z dimension position (i.e., a zoom lens).
The image is detected and reconstructed at the detector. Standard photographic film may be used for the recording of light holograms and the 3-D image reconstructed by passing laser light through the film or reflecting it from the hologram pattern embossed on the surface of an optical reflective surface. However, there is no equivalent “film” material to record the intricate phase and amplitude pattern of a complex ultrasonic wave. One of the most common detectors uses a liquid-air surface or interface to record, in a dynamic way, the ultrasonic hologram formed. The sound energy at the frequency of ultrasound (above range of human hearing) will propagate with little attenuation through a liquid (such as water) but cannot sustain substantial propagation through air. At these higher frequencies (e.g., above 1 MHz) the ultrasound will not propagate through air because the wavelength of the sound energy is so short [λ(wavelength)=v(velocity)/f(frequency)]. The density of air (approximately 0.00116 g/cm3) is not sufficient to couple these short wavelengths and allow them to propagate for any significant distance. On the other hand the density of a liquid (e.g., water) is a favorable media to couple and propagate such wavelengths. For example, the velocity of sound in air is approximately 346 meter/second whereas in water it is approximately 1497 meter/second. Thus, for water, both the density (1 g/cm3) and the wavelength (˜1.5 mm at 1 MHz) are significantly large that ultrasound can propagate with little attenuation. In contrast, for air both the density (0.00116 g/cm3) and wavelength (0.346 mm at 1 MHz) are sufficiently small such that the energy at these ultrasonic frequencies will not propagate.
Thus, when ultrasound propagating in a liquid encounters a liquid-air interface the entire amount of the energy is reflected back into the liquid. Since ultrasound (or sound) propagates as a mechanical force it is apparent that the reflection (or changing direction of propagation) will impart a forward force on this liquid-air interface. This force, in turn, will distort the surface of the liquid. The amount of surface distortion will depend upon the amplitude of the ultrasound wave at each point being reflected and the surface tension of the liquid. Thus, the pattern of the deformation is the pattern of the phase and amplitude of the ultrasonic wave at the plane (i.e. the ultrasonic hologram).
In this manner, the liquid-air interface can be readily used to provide a near real-time recorder (“film equivalent”) for an ultrasonic hologram. The shape of the surface deformation on this liquid-air detector is the representation of the phase and amplitude of the ultrasonic hologram formed by the interference of the object and reference ultrasonic waves.
The greatest value of the ultrasonic holographic process is achieved by reconstructing the hologram in a usable manner, usually in light, to make visible the structural nature of the initial object. In the case of a liquid-air interface, the reconstruction to achieve the visible image is accomplished by reflecting a coherent light from this liquid-air surface. This is the equivalent process to reflecting laser light from optically generated hologram that is embossed on the surface of a reflecting material (e.g., thin aluminum film).
The reflected light is diffracted (scattered) by the hologram to diffractive orders, each of which contains image information about the object. These diffracted orders are referred to as ±nth orders. That part of the reconstructing light that does not react with the hologram is referred to as zero order and is usually blocked so that the weaker diffracted orders can be imaged. The higher the diffracted order the greater is the separation angle between the zero order of reflected light. Once reconstructed, the image may be viewed directly, by means of a video camera or through post processing processes.
Ultrasonic holography is illustrated in prior art FIG. 1. FIG. 1 shows a plane wave of sound 12 (i.e., ultrasound) that is generated by a large area object transducer 10. One example of a large area object transducer is described in U.S. Pat. No. 5,329,202. The sound is scattered (i.e., diffracted) by structural points within the object. The scattered sound 14, from the internal object points that lie in the focal plane 16, are focused (i.e., projected) into a hologram detector plane 18 of a hologram detector 20. The focusing is accomplished by an ultrasonic lens system 22, which focuses the scattered sound into the hologram detector plane 18. According to U.S. Pat. No. 5,235,553, an ultrasonic lens is described that may be satisfactorily used for the ultrasonic lenses illustrated as the lens system 22 in FIG. 1. The ultrasonic lens system 22 also allows the imaging process to magnify the image (i.e., zoom) or change focus position. According to U.S. Pat. No. 5,212,571 a lens system is illustrated that can magnify the image and change focus position, and may be used satisfactorily for the lens system 22.
Since the focal point 24 of the unscattered sound is prior to the hologram detector plane 18, this portion of the total sound again expands to form the transparent image contribution (that portion of the sound that is transmitted through the object as if it were transparent or semi-transparent). In such an application an ultrasound reflector 26 is generally used to direct the object sound at a different angle thus impinging on the horizontal hologram detector plane 18; the hologram detector plane 18 usually contains a liquid 28 that is deformed by the ultrasound reflecting from the liquid-air interface.
When a reference wave 30 and the object wave are simultaneously reflected from the hologram detector plane 18, the deformation of the liquid-air interface is the exact pattern of the ultrasonic hologram formed by the object wave (12 combined with 14) and the “off-axis” reference wave 30.
This ultrasonic hologram formed on the detector plane 18 is subsequently reconstructed for viewing by using a coherent light source 32, which may be passed through an optical lens 34, and reflected from the holographic detector plane 18. A hologram detector suitable for use as the hologram detector 20 illustrated in FIG. 1 is described in U.S. patent application Ser. No. 09/589,863.
In the practice of ultrasonic holography an object wave is passed through or reflected from the interior or exterior structural characteristics of an object being investigated. Since this is off-axis holography, a reference wave is required to form the ultrasonic (or acoustical) hologram. Since the reference wave needs to pass unaltered from the reference transducers to the hologram area, the prior art systems required some volume or space on the ultrasonic side of the hologram that is free of the object, thus allowing an unaltered path for the reference.
These conditions required that there was some distance from the object to the hologram. This meant that the “object distance” was great; the object distance then determined the image distance. Thus, an opportunity to have a full 3-D (three dimensional) view is compromised since the aperture size to the object distance limits the 3-D information.